1. Field of the Invention
The present invention relates to a radio receiving system based on an orthogonal modulation communication method, and more particularly, to a radio receiving system which compensates for an aperture effect due to a sampling operation by setting the frequency characteristics of a band pass filterxe2x80x94which permits passage of only a signal at a frequency band assigned to a communications system from which the radio receiving system receives a signalxe2x80x94are set so as to prevent the aperture effect.
2. Description of the Related Art
A receiverxe2x80x94which is based on a direct conversion receiving method and which has a simplified radio sectionxe2x80x94is realized through use of a channel filter which samples, or subjects to an analog-to-digital conversion operation, an input signal while the input signal still remains in a high frequency state before converted into a baseband signal and which subjects the quantized signal to a stable digital signal processing operation having a high degree of accuracy. However, the channel filter suffers from the following four problems.
First, as a result of the sampling operation, the sampling frequency renders the frequency characteristics of the overall radio receiving system uneven. Consequently, a digitized signal is demodulated at a high error rate.
Second, in order to highly accurately perform a sampling operation, previous and subsequent stages of the sampling circuit must have high speed characteristics required to ensure over a considerably wide frequency range the speed performance of the sampling circuit for the purpose of preventing the aperture effect. As a result, the sampling circuit has a bandwidth which is considerably wider than the bandwidth of a received signal. In short, in spite of a band pass filter provided in a previous stage in order to limit the bandwidth of the received signal to a predetermined bandwidth, the circuit provided in the subsequent stage must have a bandwidth which is significantly wider than that of the band pass filter. Thermal noise caused by the circuit provided in the subsequent stage exceeds the amount of that caused in an existing radio receiving system, which also accounts for an increase in the error rate.
Third, under the direct conversion receiving method, there is a need to provide a base band circuit with a function as a substitute for a channel filter which is conventionally provided in an IF stage of the existing receiver. To this end, it is also necessary for an HF stage whose filtering is insufficient to maintain a wide dynamic range and a wide bandwidth. Still further, there is a need for a filter which filters a signal having such a wide dynamic range and a bandwidth.
Fourth, a sampled signal usually includes d.c. components. Since the signal becomes vulnerable to d.c. noise, drift, or offsets, the signal including such noise accounts for a large error rate in the case of a portable cellular phone based on digital modulation.
FIG. 9 shows an example of an existing direct conversion receiver which uses a bandwidth-limited sampling method. This circuit diagram corresponds to a direct IF sampling circuit used in a new produce xe2x80x9c125 MSPS Monolithic Sampling Amplifier AD9101xe2x80x9d described in xe2x80x9cAnalog Devises Converter Data Book,xe2x80x9d 1st edition, Analog Devises Co., Ltd., July, 1997. There are descriptions which state xe2x80x9cAdoption of the Nyquist theory enables elimination of an IF frequency and reconstruction of a base band signal. For example, a 40-MHz IF signal is modulated by a signal having a bandwidth of 10 MHz, and a signal to be detected is detected at a sampling rate of 25 MSPS.xe2x80x9d A 40-MHz IF signal modulated by a signal having a bandwidth of 10 MHz is usually detected at a sampling frequency which is twice as high as a frequency of 40 MHz. However, since the signal is limited to a bandwidth of 10 MHz, the IF signal can be detected at a sampling frequency of 25 MHz by utilization of the xe2x80x9cShannon""s Sampling Theoremxe2x80x9d according to which the IF signal can be sampled at a frequency twice or more as high as a frequency of 10 MHz.
FIGS. 10A to 10D are views showing variations in spectral components when direct conversion reception is performed through a bandwidth-limited sampling operation. FIG. 10A shows a desired waveform and adjacent waveforms in a radio frequency band, as well as the characteristics of a band pass filter which covers these waveforms. In the drawing, fs designates a sampling frequency set to a frequency which is twice or more as wide as a communications bandwidth or a the bandwidth of a bandwidth-limited filter.
FIG. 10B shows spectral components of the desired waveform and adjacent waveforms having frequencies converted into a baseband frequency at a sampling frequency. The baseband frequency range fBB is in principle the same as fBW.
FIG. 10C shows the result of extraction of the desired signal through a channel filtering operation, wherein a quantized signal obtained as a result of a sampling operation is subjected to a digital signal processing operation.
FIG. 10D shows the aperture effect caused by the sampling operation at this time. In other words, the drawing shows spectral components of what-is-called a sampling function. The spectral components have characteristics of {sin(xcfx80f/fs)}/(xcfx80f/fs), and a null occurs at a sampling frequency fs. Although the desired waveform in the range less than half the sampling frequency does not occur at the null point, the waveform is given the frequency characteristics which gradually attenuates a waveform toward higher frequencies.
The present invention relates to a receiver circuit having a built-in channel filter which is used with a radio receiving system assigned an offset frequency (disclosed in Japanese Patent Application Laid-open No. Hei-9-266452 xe2x80x9cReceiving System,xe2x80x9d and Japanese Patent Application No. Hei-9-28271 xe2x80x9cReceiving System,xe2x80x9d both being filed by the applicant of the present patent application) and includes a complex coefficient filter. In the channel filter including a complex coefficient filter on which the present invention is based, the center of positive and negative frequency components to be subjected to a complex operation does not necessarily occur at zero frequency. For this reason, the aperture characteristics of the channel filter which cause the center of frequency components to occur at zero frequency make the operation distorted, resulting in a considerable decrease in the accuracy of the operation. Further, even if frequency components are shaped so as to have complete Nyquist characteristics through use of a subsequent root Nyquist filter, the frequency components cannot have the complete Nyquist characteristics.
The present invention has been conceived to solve the foregoing problem in the existing radio receiving system, and the object of the present invention is to solve the problem by providing characteristics of compensating for the aperture effect due to a sampling operation to a band pass filter disposed in a receiving input stage of a receiver circuit of a radio receiving system, wherein a channel filter is realized by quantizing a received signal through sampling and by subjected the thus-quantized signal to a digitized signal processing operation.
A first aspect of the invention is directed to a radio receiving system in which a channel filter is formed by quantizing a received signal through sampling and by subjecting the thus-quantized signal to a digitized-signal processing operation, wherein a band pass filter having characteristics of compensating for the aperture effect due to a sampling operation is provided in an input receiving stage. Use of the band pass filter having the foregoing characteristics enables compensation for the aperture effect due to the sampling operation.
According to a second aspect of the invention, the radio receiving system is characterized by further comprising: a sample-and-hold circuit for sampling and holding an output from the band pass filter; and an integrating circuit having a function of integrating the received signal during a period of sampling operation of the sample-and-hold circuit. As a result of the radio receiving system being provided with the integration effect, the energy of a desired waveform signal can be integrated. Particularly, even in a case where a weak radio wave is received and a desired waveform signal is buried in thermal noise in a circuit, a sampling operation being performed at an ordinary voltage enables power to be produced from an input signal only during a period over which the aperture effect arises. However, the radio receiving system according to claim 2 has the effect of being able to double the power by integrating the received signal while the period over which the received signal is integrated is extended.
According to a third aspect of the invention, the radio receiving system as defined in the second aspect is characterized by the feature that integral action time of the integrating circuit can be changed or selected from a plurality of values. As a result, the integral action time of the sample-and-hold circuit is changed with respect to a change in the frequency or bandwidth of the input signal, enabling a desired integration effect to be accomplished.
According to a fourth aspect of the invention, the radio receiving system as defined in the third aspect is characterized by the feature that the integral capacity of the integrating circuit is made variable. As a result, the integral action time of the sample-and-hold circuit is changed with respect to a change in the frequency or bandwidth of the input signal, enabling a desired integration effect to be accomplished.
According to a fifth aspect of the invention, the radio receiving system as defined in the second aspect is characterized by the feature an integrating gate function of the integrating circuit is arranged so as to produce a Nyquist signal waveform. As a result, an efficient sample-and-hold circuit can be realized which provides a superior signal-to-noise ratio.
According to a sixth aspect of the invention, the radio receiving system as defined in either second or third aspect is characterized by the feature that the time constant of the sample-and-hold circuit is set so as to become longer than the sampling frequency. As a result, thermal noise or random signals can be sufficiently removed from a lower-frequency component. Further, diminution of the sampling frequency results in a reduction in the power dissipated by the sample-and-hold circuit or by peripheral circuits connected thereto.
According to a seventh aspect of the invention, the radio receiving system as defined in any one of the first to third aspects is characterized by further comprising: sampling means which is made of a sample-and-hold circuit and which samples the received signal; difference calculation means for calculating a difference between a currently-sampled signal received from the sample-and-hold circuit and a previously-sampled signal; and means for calculating a difference between the output from the difference calculation means and an output from the band pass filter and inputs the thus-obtained difference to the sample-and-hold circuit. As a result, the radio receiving system has the effect of being able to prevent originally-undesired components, such as temperature drift of a sampling circuit or d.c. offset of an input circuit, from being mixed into a received signal.
According to an eighth aspect of the invention, the radio receiving system as defined in any one of the first to third aspects is characterized by further comprising: sampling means which is made of a sample-and-hold circuit and which samples the received signal; Hilbert transformation means which produces rectangular components from the sample output from the sample-and-hold circuit; difference calculation means for calculating a difference between one of the rectangular components received from the transformation means and a previously-sampled rectangular component of the same type; and means for calculating a difference between the output from the difference calculation means and an output from the band pass filter and inputs the thus-obtained difference to the sample-and-hold circuit. As a result, the radio receiving system is effective in removing a d.c. component contained in a sample output produced when the sample-and-hold circuit samples a received signal together with d.c. components or originally-unwanted components mixed in the sample output such as temperature drift of the sample-and-hold circuit or d.c. offset of the input circuit.
A ninth aspect of the invention is directed to a radio receiving system which receives a signal in a direct conversion receiving mode through use of a plurality of cascaded channel filters, each including a complex coefficient filter, wherein the accuracy of operation of the preliminary channel filter is improved when compared with that of the subsequent channel filter. As a result, the radio receiving system is capable of more efficiently attenuating an adjacent waveform spaced frequencies away from a desired waveform, as well as of supplying to a filter circuit provided on a subsequent stage a signal of desired waveform from only the vicinity of which adjacent waveform signals of strong level are removed. Accordingly, even if the filter circuit provided on the subsequent stage is manufactured with less precision: that is, the filter circuit having the insufficient capability of removing waveforms of great level adjacent to the desired waveform, the radio receiving system becomes less apt to suffer from problems.
According to a tenth aspect of the invention, the radio receiving system as defined in the ninth aspect is characterized by the feature that, in order to improve the accuracy of operation of the channel filter provided in the previous stage when compared with that of the channel filter provided in the subsequent stage, the capacitance of a capacitor which is a constituent element of the channel filter provided on the subsequent stage is set so that the capacitor can be manufactured with high dimensional precision, and the capacitance of a capacitor which is a constituent element of the channel filter provided on the previous stage is set so as to become smaller than the total capacitance of the previously-described capacitors when they are cascaded. As a result, the precision of the channel filter is improved through use of capacitors having capacitance realized with the highest possible precision. Further, using capacitors having improved dimensional accuracy, a capacitor whose capacitance is smaller than the total capacitance of the capacitors can be realized.